To support air assets, the US Navy’s Altitude Laboratory in Maryland is tasked with testing On-Board Oxygen Generating Systems (OBOGS). The lab has three hypobaric chambers to assess mechanical breathing simulators at altitudes up to 60,000 feet. The altitude in the chambers is simulated by precisely regulating the vacuum pressure.
Testing oxygen generating systems presents unique challenges due to the high flow rates created by aircraft systems, the complexity of sinusoidal breathing, and the life-and-death importance of a reliable system. Previously, the lab used traditional vacuum control valves to throttle flow between hypobaric chambers and vacuum pumps. While common throughout industry, such systems are not capable of responding quickly enough to oxygen systems’ frequent perturbations in pressure/flow balance. Barometric swings of 2000 to 4000 feet were observed in some chambers.
In 2011, the Navy contracted with Research, Engineering and Development, Inc., and Spectrum Sciences, Inc., to improve the accuracy of the simulated altitude. Working with Spectrum Sciences aerospace engineer Curtis Stansfield and the Navy’s Dennis Gordge, Equilibar designed three high-flow vacuum regulators. The design used high-resolution Proportion-Air electro-pneumatic regulators to pilot-operate the three vacuum regulators. The light weight, supple diaphragm moves only a few millimeters to modulate the vacuum regulator from fully closed to fully open. The thin Viton diaphragm is sensitive to changes in pressure as small as 0.001 psi. Equilibar regulators provide stable pressure across flow rate changes of up to 1000:1 and adjust to changes in flow rate in less than 10 milliseconds.
The new system provides a significant reduction in altitude pressure variation. Even with non-steady air crew exhalation flows, the chamber altitude is maintained within +/- 50 ft, an 85% reduction in variability. The system uses one bleed orifice and two separate solenoid valves for three different modes. A very small orifice allows a low constant bleed rate for minor adjustments. A .25" sized orifice is used for controlled descent, and a .5" orifice allows for rapid ingress of air in a simulated emergency descent. The actual rate of descent in all cases is set by providing a matching ramping electronic command signal to the electronic pilot regulator.
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